1. Technical Field
This invention relates to transient suppression techniques in synchronous data detection systems. These techniques are applicable for robust gain control, timing, and data recovery in a read channel of a magnetic recording system.
2. Description of Related Art
The magnetic storage industry has been increasing the areal density storage capacity of hard drives with various technological advancements to meet the computer industry""s demand for more and better storage. Such advancements include the use of magnetoresistive (MR) heads and partial-response maximum likelihood (PRML) sequence detection. A MR head includes a MR element made out of a material which changes electrical resistance depending on the strength of the magnetic field in which it lies. A PRML system is a synchronous data detection system which includes partial-response signalling and maximum likelihood sequence detection.
MR heads are known in the industry to suffer from a transient phenomenon commonly referred to as thermal asperity (TA) events. A MR head normally glides over a spinning magnetic disk close to, but not touching, the disk surface. When a MR head hits a protruding object on the disk surface, the MR element heats up rapidly and the heat decays relatively slowly.
The effect of such a transient phenomenon is a sudden transient change in the baseline of the read-back signal coming from the MR head. This transient change contains a substantial low frequency component and causes loss of read-back data.
The extent of the read-back data lost due to such a transient depends on the robustness of the data detection system. A data detection system without any transient detection and suppression circuit may lose a large amount of data, even to the point that the error correcting code (ECC) used in the recording system cannot regenerate the user data.
A prior art method of handling a transient includes the following steps. First, the system detects the transient and halts data detection. Second, an analog front-end portion of the system is switched to high-pass mode to reduce the effect of the transient. Third, the system freezes both the gain of the gain control loop and the timing of the timing recovery loop. Fourth, the system waits for a period of time until the effect of the transient is below some threshold. This period of time is called the transient interval. Fifth, when the transient interval is over, the system resumes data detection. Sixth, the front-end is switched back to a lower AC coupling and the gain and timing are unfrozen.
There are at least two problems with the prior art method. First, even with the front-end high-pass function switched on, the transient interval is still quite long. Such a long transient interval may result in a loss of read-back data beyond the point of recovery by the ECC. Second, freezing the timing recovery and gain control loops means that the system halts tracking the timing and gain variations in the read-back signal. If the channel timing is not fully settled when the system is hit with a TA event, then freezing the timing loop will cause a large drift in the synchronous sampling points which may result in permanent synchronization loss and complete loss of user data. This problem has been shown both experimentally and through modelling.
The present invention relates to a system and method for transient suppression in a synchronous data detection system. The present invention includes high-pass filtering of the signal produced by the sampling and shaping circuits before the signal enters the timing and gain control circuits. This high-pass filtering may be turned on when a transient is detected, in anticipation of a previously detected transient, or may be always on. Using the high-pass version of the shaped signal allows the timing loop and the gain loop to function during a transient interval, thus maintaining timing and gain lock during such an interval.
This high-pass filtering of the signal entering the timing and gain control circuits facilitates the tracking of the timing and gain variations in the read-back signal. The resulting improvements in tracking capability reduces the necessary length of the transient interval which shortens the time period during which tracking is halted. A shorter time during which tracking is halted leads to less read-back data loss and smaller drift in the synchronous sampling points. Thus, loss of read-back data beyond the point of recovery by ECC and permanent synchronization loss are prevented.